Typically, a multi-site trunked communication system (system) comprises communication units, communication resources, communication sites (sites), and a communication resource allocator. Each of the sites have a substantially distinct coverage area and are geographically located throughout the system. Each site also has a number of communication resources assigned to it, where at least one of the communication resources is used as a control channel, while the remaining communication resources are used as voice channels. The communication resources may be telephone connections, TDM times slots, carrier frequencies, and/or frequency pairs.
The communication units are typically arranged into communication groups (talk groups) and may be located any where within the system (in any site). When a communication unit of a talk group requests a group call, it transmits, via a control channel of the site in which it is located, an inbound signalling word (ISW) to the communication resource allocator. (A group call typically allows all members of the same talk group that are located within the system to communicate with each other.) The ISW generally comprises the requesting communication unit's individual identification number, the requesting communication unit's talk group, and a request for a group call. The ISW may also contain more or less information depending on the system, nevertheless, the above mentioned ISW will suffice for this disclosure.
Upon receiving the ISW, the communication resource allocator will determine if there is an available voice channel in each site. If a voice channel is not available in each site, the communication resource allocator will not place the group call and will transmit an outbound signalling word (OSW) to the requesting communication unit. The OSW generally comprises, at least, a site busy signal which indicates that the group call was not placed because at least one site did not have an available voice channel. When a voice channel becomes available in each site and the requesting communication unit has not cancelled the group call request, the communication resource allocator will allocate a voice channel in each site to the requesting communication unit's talk group.
The placement of the group call may be substantially delayed if the system has a relatively high volume of calls. Under certain situations, the delay may not be acceptable. Further, in many situations, the requesting unit does not need to communicate with all the members of its talk group. For example, if an operator of the requesting communication unit is a police officer who desires to talk to a dispatcher, but no other members of his talk group (other police officers in his district), he must wait until a voice channel is available in all sites. Many systems address this problem by establishing a call priority queue which gives priority to a predetermined set of calls. However, even though the talk group has priority, it must wait until all sites have an available voice channel, which, in a system that has a relatively high volume of calls, may still be a substantial delay.
Another solution is disclosed in co-pending patent application having a Ser. No. of: 07/454,496; filing date of: Dec. 21, 1989; and is entitled: CHANNEL ASSIGNMENT METHOD FOR MULTI-SITE TRUNKED RADIO SYSTEM. In this application, the communication resource allocator only checks sites that have a member of the requesting communication unit's talk group within it. Thus, by checking only the sites having members of a talk group within it, the delay in allocating voice channels may be reduced. However, if a voice channel is not available in all the sites that have a member of the talk group within it (site busy condition), the group call will not be placed until all such sites have an available voice channel.
Therefore, a need exists for a method that will allow a requesting communication unit to override a site busy condition such that the requesting communication unit may transmit its message to as many members of the talk group as possible without having to wait for a voice channel to become available in each site having a member of the talk group within it. The need becomes even more enhanced when at least some of the sites have a relatively small number of voice channels.